1. Field of the Invention
The present invention is directed to LED (light emitting diode) arrangements. In particular, the present invention is directed to LED arrangements that can be built into a lamp housing or located in exterior lights of motor vehicles.
2. Brief Discussion of the Related Art
In the field of exterior and interior illumination of motor vehicles, light-emitting diodes (LEDs) are being increasingly used instead of conventional incandescent bulbs, particularly for tail lights and brake lights, since LEDs have a longer service life, a better efficiency in the conversion of electrical energy into radiation energy in the visible spectral range and, connected therewith, a lower heat emission and a lower space requirement overall.
EP 0 253 224 discloses a method for the manufacture of a light with light-emitting diodes. The light to be manufactured comprises a soft plastic film on whose upper side a copper lamination is applied and a plurality of light-emitting diodes are arranged. The plastic film has its side lying opposite the upper side glued onto a metallic carrier plate. The light is provided for employment in a motor vehicle, whereby the carrier plate can be implemented bent for adaptation to the shape of a motor vehicle.
Further, U.S. Pat. No. 5,782,555 discloses a traffic signal light that comprises a plurality of LEDs as luminous members. The LEDs are secured on the surface of a printed circuit board that is provided with a both-sided metallization. A plurality of through holes via which the metallizations are connected to one another are formed in the printed circuit board. The printed circuit board is secured with an adhesive to a cooling member that is provided with an electrically insulating surface.
U.S. Pat. No. 5,890,794 discloses another lighting unit on the basis of LEDs. Here, a plurality of radial LEDs is mounted on a printed circuit board, whereby the wire leads are conducted through the printed circuit board in a traditional way. In one illustrated embodiment, the printed circuit board is flexible and applied onto a cylindrical member. A coolant fluid is preferably employed for cooling.
A certain added outlay must be incurred first when constructing a light with LEDs since, due to the low luminance of an individual LED compared to an incandescent bulb, a plurality of LEDs shaped to form an array must be constructed.
For example, such an array can be mounted using surface mounting technology (SMT) with a plurality of LEDs on a printed circuit board (PCB). Such an LED structure is described in the article “SIEMENS SMT-TOPLED für die Oberflächenmontage” by F. Möllmer and G. Waitl in the periodical Siemens Components 29 (1991), Number 4, page 147. The form of the LEDs is extremely compact and allows the arrangement of a plurality of such LEDs in a row or matrix arrangement as warranted.
However, only approximately 5% of the electrical power is converted into light within the housing of such an LED that, for example, emits yellow-colored or amber-colored light, whereas approximately 95% is converted into heat. This heat is eliminated from the underside of the chip via the electrical terminal of the component. Dependent on the structure, the heat given the components known as TOPLED® or Power TOPLED® is first conducted out of the housing onto the solder points on the printed circuit board by one or three existing cathode terminals. From the solder points, the heat at first propagates in the copper pads and then on the epoxy resin material in the plane of the printed circuit board. Subsequently, the heat is output large-area to the environment by thermal radiation and thermal convection. The thermal resistance is still relatively slight in the case of a single LED on FR4 circuit board material (for example, approximately 180 K/W given an LED of the type Power TOPLED®).
The situation is different however, when many LEDs are arranged in close proximity on a circuit board. A smaller percentual area of the PCB is now available for each individual LED for heat transmission to the environment. The thermal resistance from the PCB onto the environment is correspondingly higher. For instance, given a components spacing of 6.5 mm, the thermal resistance rises to up to 550 K/W when the LEDs are of the Power TOPLED® type, and the printed circuit board is of the type FR4.
Heat is emitted from all heat-generating components on the circuit board, i.e. from the dropping resistors, transistors, MOSFETs, or drive ICs that are located in the immediate proximity of the LEDs. Operating current must be reduced so that destruction of the component does not occur as a consequence of heat generation on the circuit board and inadequate heat elimination. Thus, the luminous power of the LEDs cannot be fully exploited.
LED arrangements are utilized for the third brake light in the aforementioned field of motor vehicle lighting. This is a single-line array wherein the thermal problems are not yet so critical.